Heat tracing is often required to keep fluids flowing in pipelines. Even in a moderate climate, pipe insulation can be insufficient to maintain a fluid's temperature. For some fluids, this results in high viscosities that can reduce or prevent flow. Other potential hazards are precipitation of dissolved solids and freezing in the pipelines. In these cases, additional heat must be provided.

A thin heating strip attached to the bottom of the pipe is a common way to provide the necessary heat. This heating strip, or tracing, can be tubing filled with a hot or condensing fluid or an electrical resistance heating tape. In cases where no heat loss or thermal gradients can be tolerated, the pipe can be completely jacketed with hot or condensing fluid.

This article focuses on the technology of steam tracing systems for pipelines. It outlines design factors, safety and reliability characteristics, and discusses installation, maintenance and operating costs.

Why use steam
Steam, when generated by an on-site fossil fuel boiler, is usually the most inexpensive source of energy available in a chemical process plant. For most steam tracing systems, the steam can even be free. Since most systems can use low pressure steam, usually at 15 psi, the required steam can be produced by flashing it from process condensate (Figure 1).

Low-pressure steam can also be produced in waste heat boilers on exothermic process units (i.e., process heaters and exothermic reactors). While the boiler required to produce steam from these sources is more complex than condensate flash systems, the steam produced is still less costly than plant steam since additional fuel is not required.

Proper piping design
In a typical steam tracing system, (Figure 2), metal tubing carrying the steam is attached to the bottom of the process pipeline by metal straps. Insulation is then installed over both the pipe and the tracing tubing. For applications where high heat input is required, heat transfer cement is used to provide a more positive thermal connection between the tubing and the pipe.

Selecting the proper tracing material depends on the condensate and operating conditions. Most tracing applications use copper tubing because it is economical, bends easily, and is easy to connect using any common type of brass fittings. However, for steam pressures above 250 psi or when the condensate is corrosive, stainless steel tubing is recommended. Carbon steel tubing, in contrast, is not used because of its susceptibility to corrosion.

To prevent damage and kinking during installation and to assure unrestricted flow of steam and condensate, tubing diameter should not be smaller than 3/8 in. Smaller size tubing should be considered only for tracing runs less than 50 ft. Also, to prevent condensate draining problems the tubing must continuously slope down from the steam inlet to the trap, with no low spots.

Vertical pipes require special consideration. A single tracer along one side may not provide sufficient heat to the other side of the pipe. Coiling the tracing around the pipe (Figure 3) solves this problem, but can result in a tubing length up to 6 times the length of the pipe. The additional tubing increases the heat input and the steam pressure drop, causing the tracing temperature at the end of the run to fall below the inlet temperature.

To reduce the pressure drop, use shorter tracing runs. Also, decrease the heat input by using small diameter tubing, keeping a large distance between the wrapping turns, and using low steam pressure.

Heat tracing is often required to keep fluids flowing in pipelines. Even in a moderate climate, pipe insulation can be insufficient to maintain a fluid's temperature. For some fluids, this results in high viscosities that can reduce or prevent flow. Other potential hazards are precipitation of dissolved solids and freezing in the pipelines. In these cases, additional heat must be provided.

A thin heating strip attached to the bottom of the pipe is a common way to provide the necessary heat. This heating strip, or tracing, can be tubing filled with a hot or condensing fluid or an electrical resistance heating tape. In cases where no heat loss or thermal gradients can be tolerated, the pipe can be completely jacketed with hot or condensing fluid.